I Filled my Motorcycle Gas Tank with Diesel: Now What?


Image you ride your motorcycle to the gas station to fill up the tank. You are deep in thought and fill up the tank to the brim with diesel. Since you are on the GPS busy planning your route, you don’t notice until you turn the key and hit the starter button. You may think this will never happen to you, but apparently it is more common that you think.

A motorcycle that was filled with diesel by accident will most likely not start. Diesel requires high compression to ignite. If the motorcycle does start it will run poorly and start to smoke. Trying to start a bike on diesel is unlikely to cause damage, but it is advised to drain the tank and fuel system immediately.

Attempting to start a motorcycle that was filled with diesel by accident is not nearly as serious as trying to start a diesel engine with unleaded gas. Even though you may not damage your bike immediately, you will not be able to ride without getting dirty.

Your next step will also depend on the type of motorcycle you ride and the specific circumstances. Read on to find out what you need to do.

What Happens When I Fill My Motorcycle With Diesel?

Since the nozzle of a diesel pump will fit into the filler hole of a motorcycle gas tank, it is not uncommon for riders to accidentally fill their bike with diesel when not paying attention.

While traveling through Malawi we had to fill our bikes from drums behind the gas station due to a nationwide gas shortage. With the language barrier it was tricky to know for sure what we’re filling our bikes with. Luckily it was unleaded, but it could easily have been something else.

Selling fuel on the black market in Malawi
Who knows what those drums contain?

Either way, if you happened to fill your motorcycle with diesel and you are lucky enough to notice this, stop immediately and drain the gas tank (as explained under the next Bold heading).

Now you might be thinking, who is silly enough to make this mistake? Well, happens often enough that there are companies that specialize in coming out to drain and flush your system.

They will use specialized equipment to drain your tank and fuel lines, and clean it out with gasoline. They will also dispose of the diesel. If you don’t mind paying, this is probably the cleanest and safest way to deal with the situation but you can do it yourself.

If you managed to fill your motorcycle with diesel, chances are that you were deep in thought and will likely not notice until you hit the starter button. Your motorcycle will most likely not start at all. This is because diesel fuel will not be fully vaporized by the carburetor and it requires much higher compression to ignite than gas.

A modern, fuel injected bike’s sensors will simply tell the computer brain that something smells fishy and that it should shut down all systems. On some older bikes with a carburetor, the bike may fire up. It will probably run very rough and you may feel a loss of power as you try to pull away.

Diesel reduces the octane of the fuel and may pre-ignite. This is when the spark fires before the piston reaches the top of the combustion chamber. You will hear a knocking noise from the engine, almost like marbles being clunked around in there. Running with a knocking engine could cause serious damage due to the higher heat inside the cylinders.

If the motorcycle has a knock sensor, it will retard the timing to compensate for the lower octane gas. You will be able to ride like this for a while, but the bike will start smoking as it burns the diesel that fails to vaporize. Prolonged riding with diesel being burned may damage exhaust valves since diesel burns at a higher temperature that gas.

What Should I Do if I Accidentally Filled My Motorcycle With Diesel?

Okay, so you’ve done it. Don’t panic and don’t try to start the engine. Push your motorcycle off the forecourt and into a safe, flat area where you can get to work. You are going to drain the gas tank, fuel line, and carburetor if it is not fuel injected.

You will need a container to drain the diesel in. It obviously has to be big enough to receive the contents of your gas tank. You will also need a screw driver. If you don’t have one, borrow one from the gas station workshop or buy one from the store.

To drain the tank, follow these steps:

Step 1: Close the petcock on the gas tank

Close the petlock on your motorcycle’s gas tank. It is the small valve on the bottom of the tank where the fuel exists the tank and flows toward the engine. There will also be a reserve position (usually opposite the current open position) so make sure it is in the ‘closed’ position.

Step 2: Drain the gas tank

Place the empty container below the fuel line next to the motorcycle and with the screwdriver, remove the fuel line from the carburetor. Be ready to catch the few drops that drain from the line. You will quickly notice if you accidentally switched the petcock to reserve.

Let the diesel drain out into the container until nothing comes out. It is worth it to add a quart of unleaded to the top of the tank to flush the tank and the fuel line.

Step 3: Drain the carburetor

If your motorcycle has a carburetor (you’ll see it when you follow the fuel line down from the tank) you need to drain it next. At the bottom of the float bowl, the biggest part of the carb body, there should be a drain plug. You may need a screwdriver or a spanner to remove the drain plug. Once the float bowl is empty, replace the drain plug and the fuel line.

Step 4: Clear the combustion chamber

This step may not be necessary, especially if you noticed your mistake before you tried to start the engine. If you’ve cranked it for a few minutes before realizing your absentmindedness, some diesel may have made it into the combustion chamber. If enough diesel collect in the cylinder it may prevent it from turning over.

To be sure, remove the spark plug (you’ll need a plug spanner) and crank the engine a few times. This will displace the diesel out the plug holes. On some motorcycles, the spark plug is not that easy to get to, so you may have to remove some side covers and possibly the gas tank.

Step 5: Fill up the tank and start the engine

Make sure the carburetor drain plug is secure and that the fuel line is fastened to the float bowl inlet. Open the petcock of the gas tank and refill the tank with unleaded this time.

On fuel injected motorcycles, turning on the engine will prime the electric fuel pump and the fuel system. On a bike with a carburetor it will take some time to fill up the float bowl, so let’s hope you’ve got a good battery. Crank the engine until system is primed and it should fire up.

Don’t worry if you didn’t get all the diesel out. A small amount of diesel mixed with a full tank of unleaded gas won’t hurt your motorcycle.

Will I Need Long Term Repairs?

If you’ve drained the tank immediately after noticing your mistake you should not have to worry about lasting issues. Running the engine for a while with normal unleaded will clear the system and by the time you fill up again everything should be clean.

If your bike fired up initially and you managed to ride with the diesel in the tank, followed by a plume of smoke, I recommend you change the engine oil. The diesel that entered your combustion chamber won’t fully ignite and burn off, and some of it may end up in your oil sump. If this happens, the gas will dilute the oil, reduce its viscosity and lower the oil pressure.

This could starve the bearings and cause premature engine wear or even catastrophic engine failure. Again, this is unlikely if you only briefly tried to start it and realized something is wrong.

Conclusion

If you filled your motorcycle with diesel and immediately noticed your mistake it won’t do any harm. If you try to start the bike, it will most likely not run. The chances of causing serious damage is slim and you can simply drain the diesel and refill with gas.

Learn from your mistake and pay attention at the gas station next time. And if it happens to you one day, don’t panic! You know what to do.

Francois Steyn

I've been riding motorcycles since I was in school and have traveled thousands of miles on various bikes through more than 10 countries. For more info, check out my about page: https://adventurebiketroop.com/about-us/

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